Exhaust gas purification system

ABSTRACT

An exhaust purification system includes: an NOx reduction type catalyst, which is provided in an exhaust system; a temperature acquisition unit, which acquires a catalyst temperature of the NOx reduction type catalyst; and a regeneration treatment unit, which executes a catalyst regeneration to recover an NOx purification capacity, wherein the regeneration treatment unit alternately executes a rich control, in which an exhaust air fuel ratio is set to a rich state to raise a temperature of the NOx reduction type catalyst to a predetermined target temperature, and a lean control, in which the exhaust air fuel ratio is set to a lean state to lower the temperature of the NOx reduction type catalyst, and sets an execution period of the lean control based on a deviation between the catalyst temperature acquired by the temperature acquisition unit during the previous rich control and the target temperature.

TECHNICAL FIELD

The present invention relates to an exhaust purification system.

BACKGROUND ART

In the background art, an NOx occlusion reduction type catalyst is known as a catalyst which reduces and purifies a nitrogen compound (NOx) in an exhaust gas discharged from an internal combustion engine. When the exhaust gas is under a lean atmosphere, the NOx occlusion reduction type catalyst occludes the NOx contained in the exhaust gas. When the exhaust gas is under a rich atmosphere, the NOx occlusion reduction type catalyst detoxifies the occluded NOx through reducing and purifying by hydrocarbon contained in the exhaust gas, and discharges the NOx.

In the NOx occlusion reduction type catalyst, a sulfur oxide contained in the exhaust gas (hereinafter, referred to as SOx) is also occluded. There is a problem that when the SOx occlusion amount increases, the NOx purification capacity of the NOx occlusion reduction type catalyst is reduced. For this reason, in a case where an SOx occlusion amount reaches a predetermined amount, in order that the SOx is desorbed from the NOx occlusion reduction type catalyst to recover the NOx occlusion reduction type catalyst from S-poisoning, it is necessary to regularly perform the so-called SOx purge in which an unburned fuel is supplied to an upstream-side oxidation catalyst by the post injection or the exhaust pipe injection to raise an exhaust temperature to an SOx desorption temperature (for example, see Patent Literature 1).

CITATION LIST Patent Literature

-   [Patent Literature 1]: Japanese Unexamined Patent Application     Publication No. 2009-047086 -   [Patent Literature 2]: Japanese Unexamined Patent Application     Publication No. 2007-315225 -   [Patent Literature 3]: Japanese Unexamined Patent Application     Publication No. 2009-115038

SUMMARY OF INVENTION Technical Problem

A method, in which a rich control to set an exhaust air fuel ratio to a rich state so as to raise the exhaust temperature and a lean control to set an exhaust air fuel ratio to a lean state so as to lower the exhaust temperature are alternately executed, is known as a method in which a catalyst temperature at the time of the SOx purge is kept in a predetermined temperature range (for example, see Patent Literature 1). However, if respective execution periods of the rich control and the lean control are not optimally controlled, the exhaust temperature is excessively raised during the rich control, which may cause heat deterioration of the NOx occlusion reduction type catalyst. In addition, when the exhaust temperature is excessively lowered during the lean control, the catalyst temperature may be hardly stabilized to the SOx desorption temperature.

The disclosed system is made to effectively suppress that a catalyst temperature at the time of an SOx purge is excessively raised or lowered.

The disclosed system is an exhaust purification system including an NOx reduction type catalyst, which is provided in an exhaust system of an internal combustion engine and reduces and purifies NOx in an exhaust gas; a temperature acquisition unit, which acquires a catalyst temperature of the NOx reduction type catalyst; and a regeneration treatment unit, which executes a catalyst regeneration to recover an NOx purification capacity of the NOx reduction type catalyst, wherein the regeneration treatment unit alternately executes a rich control, in which an exhaust air fuel ratio is set to a rich state so as to raise the NOx reduction type catalyst to a predetermined target temperature, and a lean control, in which the exhaust air fuel ratio is set to a lean state so as to lower a temperature of the NOx reduction type catalyst, and sets an execution period of the lean control by a PID control, based on a deviation between a catalyst temperature acquired by the temperature acquisition unit during the previous rich control and the target temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an entire configuration diagram illustrating an exhaust purification system according to this embodiment.

FIG. 2 is a timing chart for describing an SOx purge control according to this embodiment.

FIG. 3 is a block diagram illustrating a setting process of a MAF target value at the time of an SOx purge lean control according to this embodiment.

FIG. 4 is a block diagram illustrating a setting process of a target injection amount at the time of an SOx purge rich control according to this embodiment.

FIG. 5 is a timing chart for describing a catalyst temperature adjustment control in the SOx purge control according to this embodiment.

FIG. 6 is a timing chart for describing an NOx purge control according to this embodiment.

FIG. 7 is a block diagram illustrating a setting process of a MAF target value at the time of an NOx purge lean control according to this embodiment.

FIG. 8 is a block diagram illustrating a setting process of a target injection amount at the time of an NOx purge rich control according to this embodiment.

FIG. 9 is a block diagram illustrating a process of an injection amount learning correction of an injector according to this embodiment.

FIG. 10 is a flow diagram for describing a calculation process of a learning correction coefficient according to this embodiment.

FIG. 11 is a block diagram illustrating a setting process of a MAF correction coefficient according to this embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an exhaust purification system according to one embodiment of the present invention will be described based on accompanying drawings.

As illustrated in FIG. 1, an injector 11 which directly injects high pressure fuel accumulated in a common rail (not illustrated) into a cylinder is provided in each of cylinders of a diesel engine (hereinafter, simply referred to as an engine) 10. The fuel injection amount or the fuel injection timing of the injector 11 is controlled in response to an instruction signal input from an electronic controller (hereinafter, referred to as ECU) 50.

An intake manifold 10A of the engine 10 is connected with an intake passage 12 which introduces fresh air therein, and an exhaust manifold 10B is connected with an exhaust passage 13 which derives an exhaust gas outside. An air cleaner 14, an intake air amount sensor (hereinafter, referred to as a MAF sensor) 40, a compressor 20A of a variable capacity supercharger 20, an intercooler 15, an intake throttle valve 16, and the like are provided in order from an intake upstream side in the intake passage 12. A turbine 20B of the variable capacity supercharger 20, an exhaust post-treatment device 30, and the like are provided in order from an exhaust upstream side in the exhaust passage 13. In FIG. 1, a reference numeral 41 denotes an engine speed sensor, a reference numeral 42 denotes an accelerator opening sensor, and a reference numeral 46 denotes a boost pressure sensor.

An EGR device 21 includes an EGR passage 22 which connects the exhaust manifold 10B and the intake manifold 10A, an EGR cooler 23 which cools an EGR gas, and an EGR valve 24 which adjusts an EGR amount.

The exhaust post-treatment device 30 is configured such that an oxidation catalyst 31, an NOx occlusion reduction type catalyst 32, and a particulate filter (hereinafter, simply referred to as a filter) 33 are disposed in order from the exhaust upstream side in a case 30A. An exhaust pipe injection device 34 which injects an unburned fuel (mainly, HC) into the exhaust passage 13 in response to the instruction signal input from an ECU 50 is provided in the exhaust passage 13 on the upstream side from the oxidation catalyst 31.

For example, the oxidation catalyst 31 is formed by carrying an oxidation catalyst component on a ceramic carrier surface such as a honeycomb structure. When an unburned fuel is supplied by the post injection of the exhaust pipe injection device 34 or the injector 11, the oxidation catalyst 31 oxidizes the unburned fuel to raise the exhaust temperature.

For example, the NOx occlusion reduction type catalyst 32 is formed by carrying an alkali metal and the like on a ceramic carrier surface such as a honeycomb structure. The NOx occlusion reduction type catalyst 32 occludes NOx in the exhaust gas when an exhaust air fuel ratio is in a lean state, and reduces and purifies the occluded NOx by a reducing agent (HC and the like) contained in the exhaust gas when the exhaust air fuel ratio is in a rich state.

For example, the filter 33 is formed such that a plurality of cells sectioned by porous partition walls are disposed in a flowing direction of the exhaust gas, and the upstream side and the downstream side of the cells are sealed alternately. In the filter 33, PM in the exhaust gas is collected in a pore or a surface of the partition wall, and when the estimation amount of PM deposition reaches a predetermined amount, the so-called filter-forced regeneration is performed which combusts and removes the PM. The filter-forced regeneration is performed in such a manner that the unburned fuel is supplied to the oxidation catalyst 31 on the upstream side by an exhaust pipe injection or the post injection, and the temperature of the exhaust gas flowing in the filter 33 is raised to a PM combusting temperature.

A first exhaust temperature sensor 43 is provided on the upstream side from the oxidation catalyst 31, and detects the temperature of the exhaust gas flowing in the oxidation catalyst 31. A second exhaust temperature sensor 44 is provided between the oxidation catalyst 31 and the NOx occlusion reduction type catalyst 32, and detects the temperature of the exhaust gas flowing in the NOx occlusion reduction type catalyst 32. An NOx/lambda sensor 45 is provided on the downstream side from the filter 33, and detects an NOx value and a lambda value of the exhaust gas passing through the NOx occlusion reduction type catalyst 32 (hereinafter, referred to as an excess-air-ratio).

The ECU 50 performs various controls on the engine 10 and the like, and includes a well-known CPU or a ROM, a RAM, an input port, an output port, and the like. In order to perform the various controls, the sensor values of the sensors 40 to 45 are input to the ECU 50. The ECU 50 includes a filter-forced regeneration controller 51, an SOx desorption treatment unit 60, an NOx desorption treatment unit 70, a MAF follow-up controller 80, an injection amount learning correction unit 90, and a MAF correction coefficient calculation unit 95 as partial functional elements. In description, such functional elements are included in the ECU 50 which is an integral hardware. However, any part thereof may be provided in a separate hardware.

<Filter-Forced Regeneration Control>

The filter-forced regeneration controller 51 estimates the PM deposition amount of the filter 33 from the travel distance of the vehicle, or the differential pressure across the filter detected by a differential pressure sensor (not illustrated), and turns on a forced regeneration flag F_(DPF) when the estimation amount of PM deposition exceeds a predetermined upper limit threshold (see time t₁ of FIG. 2). When the forced regeneration flag F_(DPF) is turned on, the instruction signal which executes the exhaust pipe injection is transmitted to the exhaust pipe injection device 34, or the instruction signal which executes the post injection is transmitted to each of the injectors 11, so that the exhaust temperature is raised to the PM combusting temperature (for example, about 550° C.). The forced regeneration flag F_(DPF) is turned off when the estimation amount of PM deposition is reduced to a predetermined lower limit threshold (determination threshold) indicating combusting and removing (see time t₂ of FIG. 2). For example, the determination threshold in which the forced regeneration flag F_(DPF) is turned off may be set based on the upper limit elapsed time or the upper limit cumulative injection amount from the start (F_(DPF)=1) of the filter-forced regeneration.

<SOx Purge Control>

The SOx desorption treatment unit 60 is an example of a regeneration treatment unit of the present invention, and executes a control (hereinafter, referred to the control as an SOx purge control) which recovers the NOx occlusion reduction type catalyst 32 from SOx-poisoning by setting the exhaust gas to a rich state so as to raise the exhaust temperature to a sulfur desorption temperature (for example, about 600° C.).

FIG. 2 illustrates a timing flowchart of the SOx purge control of this embodiment. As illustrated in FIG. 2, the SOx purge flag F_(SP) which starts the SOx purge control is turned on simultaneously when the forced regeneration flag F_(DPF) is turned off (see time t₂ of FIG. 2). Accordingly, a transition to the SOx purge control can be efficiently performed from a state where the exhaust temperature is raised by the forced regeneration of the filter 33, and the fuel consumption amount can be reduced effectively.

In this embodiment, the enrichment of the exhaust gas is made by using the SOx purge control, for example, in a such a manner that the SOx purge lean control that lowers the excess-air-ratio by an air-system control from a steady operating state (for example, about 1.5) to a first target excess-air-ratio (for example, about 1.3) on a lean side from a value equivalent to a theoretical air-fuel ratio (about 1.0), and the SOx purge rich control that lowers the excess-air-ratio by the injection control from the first target excess-air-ratio to a second target excess-air-ratio on a rich side (for example, about 0.9) are used in combination. Hereinafter, a detail description will be given about the SOx purge lean control and the SOx purge rich control.

<Air-System Control of SOx Purge Lean Control>

FIG. 3 is a block diagram illustrating a setting process of a MAF target value MAF_(SPL Trgt) at the time of the SOx purge lean control. A first target excess-air-ratio setting map 61 is a map based on an engine speed Ne and an accelerator opening degree Q (fuel injection amount of the engine 10). An excess-air-ratio target value λ_(SPL Trgt) (first target excess-air-ratio) at the time of the SOx purge lean control corresponding to the engine speed Ne and the accelerator opening degree Q is set based on an experiment and the like, in advance.

First, the excess-air-ratio target value λ_(SPL Trgt) at the time of the SOx purge lean control is read from the first target excess-air-ratio setting map 61 by using the engine speed Ne and the accelerator opening degree Q as input signals, and is input to a MAF target value calculation unit 62. In addition, in the MAF target value calculation unit 62, the MAF target value MAF_(SPL Trgt) at the time of the SOx purge lean control is calculated based on the following Equation (1).

MAF _(SPL Trgt)=λ_(SPL Trgt) ×Q _(fnl corrd) ×Ro _(Fuel) ×AFR _(sto) /Maf _(corr)  (1)

In Equation (1), Q_(fnl corrd) indicates a learning-corrected (to be described later) fuel injection amount (excluding the post injection), Ro_(Fuel) indicates a fuel specific gravity, AFR_(sto) indicates a theoretical air-fuel ratio, and Maf_(corr) indicates a MAF correction coefficient (to be described later).

The MAF target value MAF_(SPL Trgt) calculated by the MAF target value calculation unit 62 is input to a ramp treatment unit 63 when the SOx purge flag F_(SP) is turned on (see time t₂ of FIG. 2). The ramp treatment unit 63 reads a ramp coefficient from ramp coefficient maps 63A and 63B by using the engine speed Ne and the accelerator opening degree Q as input signals, and inputs a MAF target ramp value MAF_(SPL Trgt Ramp), in which the ramp coefficient is added, to a valve controller 64.

The valve controller 64 executes a feedback control that throttles the intake throttle valve 16 to the shutting side and opens the EGR valve 24 to the open side such that an actual MAF value MAF_(Act) input from the MAF sensor 40 becomes the MAF target ramp value MAF_(SPL Trgt Ramp).

In this manner, in this embodiment, the MAF target value MAF_(SPL Trgt) is set based on the excess-air-ratio target value λ_(SPL Trgt) read from the first target excess-air-ratio setting map 61 and the fuel injection amount of the injector 11, and an air system operation is feedback-controlled based on the MAF target value MAF_(SPL Trgt). Accordingly, without providing the lambda sensor on the upstream side of the NOx occlusion reduction type catalyst 32 or without using a sensor value of the lambda sensor even when the lambda sensor is provided on the upstream side of the NOx occlusion reduction type catalyst 32, the exhaust gas can be effectively lowered to the desired excess-air-ratio required for the SOx purge lean control.

When the fuel injection amount Q_(fnl corrd) after the learning correction is used as the fuel injection amount of the injector 11, the MAF target value MAF_(SPL Trgt) can be set by a feed-forward control to effectively exclude influence such as the aged deterioration, the property change, or the individual difference of the injector 11.

When the ramp coefficient set in response to the operating state of the engine 10 is added to the MAF target value MAF_(SPL Trgt), the deterioration of the drivability and the like caused by the misfire or the torque fluctuation of the engine 10 due to the rapid change of an intake air amount can be effectively suppressed.

<Fuel Injection Amount Setting of SOx Purge Rich Control>

FIG. 4 is a block diagram illustrating a setting process of the target injection amount Q_(SPR Trgt) (injection amount per unit of time) of the exhaust pipe injection or the post injection in the SOx purge rich control. A second target excess-air-ratio setting map 65 is a map based on the engine speed Ne and the accelerator opening degree Q. The excess-air-ratio target value λ_(SPR Trgt) (second target excess-air-ratio) at the time of the SOx purge rich control corresponding to the engine speed Ne and the accelerator opening degree Q is set based on an experiment and the like, in advance.

First, the excess-air-ratio target value λ_(SPR Trgt) at the time of the SOx purge rich control is read from the second target excess-air-ratio setting map 65 by using the engine speed Ne and the accelerator opening degree Q as input signals, and is input to an injection amount target value calculating unit 66. In addition, in the injection amount target value calculating unit 66, the target injection amount Q_(SPR Trgt) at the time of the SOx purge rich control is calculated based on the following Equation (2).

Q _(SPR Trgt) =MAF _(SPL Trgt) ×Maf _(corr)/(λ_(SPR Target) ×Ro _(Fuel) ×AFR _(sto))−Q _(fnl corrd)  (2)

In Equation (2), MAF_(SPL Trgt) is a MAF target value at the time of a lean SOx purge, and is input from the above-described MAF target value calculation unit 62. Q_(fnlRaw corrd) indicates a learning-corrected (to be described later) fuel injection amount (excluding the post injection) before a MAF follow-up control is applied thereto, Ro_(Fuel) indicates a fuel specific gravity, and AFR_(sto) indicates a theoretical air-fuel ratio, and Maf_(corr) indicates a MAF correction coefficient (to be described later).

When the SOx purge rich flag F_(SPR) (to be described later) is turned on, the target injection amount Q_(SPR Trgt) calculated by the injection amount target value calculating unit 66 is transmitted as the injection instruction signal to the exhaust pipe injection device 34 or the injector 11.

In this manner, in this embodiment, the target injection amount Q_(SPR Trgt) is set based on the excess-air-ratio target value λ_(SPR Trgt) read from the second target excess-air-ratio setting map 65 and the fuel injection amount of the injector 11. Accordingly, without providing the lambda sensor on the upstream side of the NOx occlusion reduction type catalyst 32 or without using a sensor value of the lambda sensor even when the lambda sensor is provided on the upstream side of the NOx occlusion reduction type catalyst 32, the exhaust gas can be effectively lowered to the desired excess-air-ratio required for the SOx purge rich control.

When the fuel injection amount Q_(fnl corrd) after the learning correction is used as the fuel injection amount of the injector 11, the target injection amount Q_(SPR Trgt) can be set by the feed-forward control to effectively exclude influence such as the aged deterioration, the property change, or the like of the injector 11.

<Catalyst Temperature Adjustment Control of SOx Purge Control>

As illustrated in times t₂ to t₄ of FIG. 2, the temperature of the exhaust gas (hereinafter, referred to as a catalyst temperature) flowing in the NOx occlusion reduction type catalyst 32 during the SOx purge control is controlled by alternately switching on and off (rich and lean) of the SOx purge rich flag F_(SPR) which executes the exhaust pipe injection or the post injection. When the SOx purge rich flag F_(SPR) is turned on (F_(SPR)=1), the catalyst temperature is raised by the exhaust pipe injection or the post injection (hereinafter, referred to a time thereof as an injection time T_(F INJ)). On the other hand, when the SOx purge rich flag F_(SPR) is turned off, the catalyst temperature is lowered by the stop of the exhaust pipe injection or the post injection (hereinafter, referred to a time thereof as an interval T_(F INT)).

In this embodiment, the injection time T_(F INJ) is set by reading a value corresponding to the engine speed Ne and the accelerator opening degree Q from an injection time setting map (not illustrated) created through an experiment and the like, in advance. In the injection time setting map, the injection time required to reliably lower the excess-air-ratio of the exhaust gas obtained by an experiment and the like, in advance to the second target excess-air-ratio is set in response to the operating state of the engine 10.

When the SOx purge rich flag F_(SPR) in which the catalyst temperature is the highest is switched from the On state to the Off state, the interval T_(F INT) is set through a feedback control. Specifically, the interval T_(F INT) is processed by a PID control configured by a proportional control that changes an input signal in proportion to the deviation ΔT between a target catalyst temperature and an estimated catalyst temperature when the SOx purge rich flag F_(SPR) is turned off, an integral control that changes the input signal in proportion to a time integral value of the deviation ΔT, and a differential control that changes the input signal in proportion to a time differential value of the deviation ΔT. The target catalyst temperature is set to such a degree as to desorb SOx from the NOx occlusion reduction type catalyst 32. The estimated catalyst temperature may be estimated, for example, based on an inlet temperature of the oxidation catalyst 31 detected by the first exhaust temperature sensor 43, an exothermic reaction inside the oxidation catalyst 31 and the NOx occlusion reduction type catalyst 32, and the like.

As illustrated in time t₁ of FIG. 5, when the SOx purge flag F_(SP) is turned on by the termination of the filter-forced regeneration (F_(DPF)=0), the SOx purge rich flag F_(SPR) is also turned on, and the interval T_(F INT) feedback-calculated at the time of the previous SOx purge control is reset temporarily. That is, at first time just after the filter-forced regeneration, the exhaust pipe injection or the post injection is executed in response to the injection time T_(F INJ 1) set in the injection time setting map (see time from t₁ to t₂ of FIG. 5). In this manner, the SOx purge control starts from the SOx purge rich control without performing the SOx purge lean control, and thus a prompt transition to the SOx purge control can be performed and the fuel consumption amount can be reduced without lowering the exhaust temperature raised by the filter-forced regeneration.

Next, when the SOx purge rich flag F_(SPR) is turned off with the lapse of the injection time T_(F INJ 1), the SOx purge rich flag F_(SPR) is turned off until the interval T_(F INT 1) set by the PID control elapses (see times t₂ to t₃ of FIG. 5). In addition, when the SOx purge rich flag F_(SPR) is turned on with the lapse of the interval T_(F INT 1), the exhaust pipe injection or the post injection according to the injection time T_(F INJ 2) is executed again (see time from t₃ to t₄ of FIG. 5). Thereafter, the on-and-off switching of the SOx purge rich flag F_(SPR) is repeatedly executed until the SOx purge flag F_(SP) is turned off (see time t_(n) of FIG. 5) by the termination determination of the SOx purge control (to be described later).

In this manner, in this embodiment, the injection time T_(F INJ) in which the catalyst temperature is raised and the excess-air-ratio is lowered to the second target excess-air-ratio is set from the map based on the operating state of the engine 10, and the interval T_(F INT) in which the catalyst temperature is lowered is treated by the PID control. Accordingly, the catalyst temperature in the SOx purge control is effectively kept in the desired temperature range required for a purge, and the excess-air-ratio can be reliably lowered to a target excess ratio.

<Termination Determination of SOx Purge Control>

When any condition of (1) a case where the injection amount of the exhaust pipe injection or the post injection is accumulated since the SOx purge flag F_(SP) is turned on and then the cumulative injection amount reaches a predetermined upper limit threshold amount, (2) a case where the elapsed time timed from the start of the SOx purge control reaches a predetermined upper limit threshold time, and (3) a case where the SOx adsorbing amount of the NOx occlusion reduction type catalyst 32 calculated based on a predetermined model equation including an operating state of the engine 10, a sensor value of the NOx/lambda sensor 45, or the like as input signals is reduced to a predetermined threshold indicating SOx removal success is satisfied, the SOx purge control is terminated by turning off the SOx purge flag F_(SP) (see time t₄ of FIG. 2 and time t_(n) of FIG. 5).

In this manner, in this embodiment, the upper limit of the cumulative injection amount and the elapsed time is set in the termination condition of the SOx purge control, so that it can be effectively suppressed that the fuel consumption amount is excessive in a case where the SOx purge does not progress due to the lowering of the exhaust temperature and the like.

<NOx Purge Control>

The NOx desorption treatment unit 70 is an example of the regeneration treatment unit of the present invention. The NOx desorption treatment unit 70 executes a control that recovers the NOx occlusion capacity of the NOx occlusion reduction type catalyst 32 by detoxifying the NOx, which is occluded in the NOx occlusion reduction type catalyst 32 when the exhaust gas is under a rich atmosphere, by reducing and purifying, and then discharging the NOx (hereinafter, referred to the control as an NOx purge control).

The NOx purge flag F_(NP) which starts the NOx purge control is turned on when an NOx discharging amount per unit of time is estimated from the operating state of the engine 10 and then an estimated accumulated value ΣNOx calculated by accumulating the NOx discharging amounts exceeds the predetermined threshold (see time t₁ of FIG. 6). Alternatively, the NOx purge flag F_(NP) is turned on in a case where an NOx purification rate of the NOx occlusion reduction type catalyst 32 is calculated from the NOx discharging amount on the catalyst upstream side estimated from the operating state of the engine 10 and then an NOx amount on the catalyst downstream side detected by the NOx/lambda sensor 45, and the NOx purification rate is lower than the predetermined determination threshold.

In this embodiment, the enrichment of the exhaust gas is made by using the NOx purge control, for example, in such a manner that the NOx purge lean control that lowers the excess-air-ratio by an air-system control from a steady operating state (for example, about 1.5) to a third target excess-air-ratio (for example, about 1.3) on a lean side from a value equivalent to a theoretical air-fuel ratio (about 1.0), and the NOx purge rich control that lowers the excess-air-ratio by the injection control from a fourth target excess-air-ratio to the second target excess-air-ratio on a rich side (for example, about 0.9) are used in combination. Hereinafter, the detail description will be given about the NOx purge lean control and the NOx purge rich control.

<MAF Target Value Setting of NOx Purge Lean Control>

FIG. 7 is a block diagram illustrating a setting process of the MAF target value MAF_(NPL Trgt) at the time of the NOx purge lean control. A third target excess-air-ratio setting map 71 is a map based on the engine speed Ne and the accelerator opening degree Q. The excess-air-ratio target value λ_(NPL Trgt) (third target excess-air-ratio) at the time of the NOx purge lean control corresponding to the engine speed Ne and the accelerator opening degree Q is set based on an experiment and the like, in advance.

First, the excess-air-ratio target value λ_(NPL Trgt) at the time of the NOx purge lean control is read from the third target excess-air-ratio setting map 71 by using the engine speed Ne and the accelerator opening degree Q as input signals, and is input to the MAF target value calculation unit 72. In addition, in the MAF target value calculation unit 72, the MAF target value MAF_(NPL Trgt) at time of the NOx purge lean control is calculated based on the following Equation (3).

MAF _(NPL Trgt)=λ_(NPL Trgt) ×Q _(fnl corrd) ×Ro _(Fuel) ×AFR _(sto) /Maf _(corr)  (3)

In Equation (3), Q_(fnl corr) indicates a learning-corrected (to be described later) fuel injection amount (excluding the post injection), Ro_(Fuel) indicates a fuel specific gravity, AFR_(sto) indicates a theoretical air-fuel ratio, and Maf_(corr) indicates a MAF correction coefficient (to be described later).

The MAF target value MAF_(NPL Trgt) calculated by the MAF target value calculation unit 72 is input to a ramp treatment unit 73 when the NOx purge flag F_(SP) is turned on (see time t₁ of FIG. 6). The ramp treatment unit 73 reads a ramp coefficient from ramp coefficient maps 73A and 73B by using the engine speed Ne and the accelerator opening degree Q as input signals, and inputs a MAF target ramp value MAF_(NPL Trgt Ramp), in which the ramp coefficient is added, to a valve controller 74.

The valve controller 74 executes a feedback control that throttles the intake throttle valve 16 to the shutting side and opens the EGR valve 24 to the open side such that the actual MAF value MAF_(Act) input from the MAF sensor 40 becomes the MAF target ramp value MAF_(NPL Trgt Ramp).

In this manner, in this embodiment, the MAF target value MAF_(NPL Trgt) is set based on the excess-air-ratio target value λ_(NPL Trgt) read from the third target excess-air-ratio setting map 71 and the fuel injection amount of the injector 11, and an air system operation is feedback-controlled based on the MAF target value MAF_(NPL Trgt). Accordingly, without providing the lambda sensor on the upstream side of the NOx occlusion reduction type catalyst 32 or without using a sensor value of the lambda sensor even when the lambda sensor is provided on the upstream side of the NOx occlusion reduction type catalyst 32, the exhaust gas can be effectively lowered to the desired excess-air-ratio required for the NOx purge lean control.

When the fuel injection amount Q_(fnl corrd) after the learning correction is used as the fuel injection amount of the injector 11, the MAF target value MAF_(NPL Trgt) can be set by a feed-forward control to effectively exclude influence such as the aged deterioration, the property change, or the like of the injector 11.

When the ramp coefficient set in response to the operating state of the engine 10 is added to the MAF target value MAF_(NPL Trgt), the deterioration of the drivability and the like caused by the misfire or the torque fluctuation of the engine 10 due to the rapid change of the intake air amount can be effectively suppressed.

<Fuel Injection Amount Setting of NOx Purge Rich Control>

FIG. 8 is a block diagram illustrating a setting process of the target injection amount Q_(NPR Trgt) (injection amount per unit of time) of the exhaust pipe injection or the post injection in the NOx purge rich control. A fourth target excess-air-ratio setting map 75 is a map based on the engine speed Ne and the accelerator opening degree Q. The excess-air-ratio target value λ_(NPR Trgt) (fourth target excess-air-ratio) at the time of the NOx purge rich control corresponding to the engine speed Ne and the accelerator opening degree Q is set based on an experiment and the like, in advance.

First, the excess-air-ratio target value λ_(NPR Trgt) at the time of the NOx purge rich control is read from the fourth target excess-air-ratio setting map 75 by using the engine speed Ne and the accelerator opening degree Q as input signals, and is input to an injection amount target value calculating unit 76. In addition, in the injection amount target value calculating unit 76, the target injection amount Q_(NPR Trgt) at the time of the NOx purge rich control is calculated based on the following Equation (4).

Q _(NPR Trgt) =MAF _(NPL Trgt) ×Maf _(corr)(λ_(NPR Target) ×Ro _(Fuel) ×AFR _(sto))−Q _(fnl corrd)  (4)

In Equation (4), MAF_(NPL Trgt) is a MAF target value at the time of a lean NOx purge, and is input from the above-described MAF target value calculation unit 72. Q_(fullRaw corrd) indicates a learning-corrected (to be described later) fuel injection amount (excluding the post injection) before a MAF follow-up control is applied thereto, Ro_(Fuel) indicates a fuel specific gravity, and AFR_(sto) indicates a theoretical air-fuel ratio, and Maf_(corr) indicates a MAF correction coefficient (to be described later).

When the NOx purge flag F_(SP) is turned on, the target injection amount Q_(NPR Trgt) calculated by the injection amount target value calculating unit 76 is transmitted as the injection instruction signal to the exhaust pipe injection device 34 or the injector 11 (time t₁ of FIG. 6). The transmission of the injection instruction signal is continued until the NOx purge flag F_(NP) is turned off (time t₂ of FIG. 6) by the termination determination of the NOx purge control (to be described later).

In this manner, in this embodiment, the target injection amount Q_(NPR Trgt) is set based on the excess-air-ratio target value λ_(NPR Trgt) read from the fourth target excess-air-ratio setting map 75 and the fuel injection amount of the injector 11. Accordingly, without providing the lambda sensor on the upstream side of the NOx occlusion reduction type catalyst 32 or without using a sensor value of the lambda sensor even when the lambda sensor is provided on the upstream side of the NOx occlusion reduction type catalyst 32, the exhaust gas can be effectively lowered to the desired excess-air-ratio required for the NOx purge rich control.

When the fuel injection amount Q_(fnl corrd) after the learning correction is used as the fuel injection amount of the injector 11, the target injection amount Q_(NPR Trgt) can be set by the feed-forward control to effectively exclude influence such as the aged deterioration, the property change, or the like of the injector 11.

<Air-System Control Prohibition of NOx Purge Control>

In an area where the operating state of the engine 10 is in a low load, the ECU 50 feedback-controls the opening degree of the intake throttle valve 16 or the EGR valve 24 based on a sensor value of the MAF sensor 40. On the other hand, in an area where the operating state of the engine 10 is in a high load, the ECU 50 feedback-controls a supercharging pressure by the variable capacity supercharger 20 based on a sensor value of the boost pressure sensor 46 (hereinafter, referred to the area as a boosting pressure FB control area).

In such a boosting pressure FB control area, a phenomenon occurs in which the control of the intake throttle valve 16 or the EGR valve 24 interferes with the control of the variable capacity supercharger 20. For this reason, there is a problem that the intake air amount cannot be kept to the MAF target value MAF_(NPL Trgt) even when the NOx purge lean control is executed in which air system is feedback-controlled based on the MAF target value MAF_(NPL Trgt) set in the above-described Equation (3). As a result, even when the NOx purge rich control to execute the post injection or the exhaust pipe injection starts, the excess-air-ratio may not be lowered to the fourth target excess-air-ratio (excess-air-ratio target value λ_(NPR Trgt)) required for the NOx purge.

In order to avoid such a phenomenon, in the boosting pressure FB control area, the NOx desorption treatment unit 70 of this embodiment prohibits the NOx purge lean control to adjust the opening degree of the intake throttle valve 16 or the EGR valve 24, and lowers the excess-air-ratio to the fourth target excess-air-ratio (excess-air-ratio target value λ_(NPR Trgt)) only through the exhaust pipe injection or the post injection. Accordingly, even in the boosting pressure FB control area, the NOx purge can be performed reliably. In addition, in the case, the MAF target value set based on the operating state of the engine 10 may be applied to the MAF target value MAF_(NPL Trgt) of the above-described Equation (4).

<Termination Determination of NOx Purge Control>

When any condition of (1) a case where the injection amount of the exhaust pipe injection or the post injection is accumulated since the NOx purge flag F_(NP) is turned on and then the cumulative injection amount reaches a predetermined upper limit threshold amount, (2) a case where the elapsed time timed from the start of the NOx purge control reaches the predetermined upper limit threshold time, and (3) a case where the NOx occlusion amount of the NOx occlusion reduction type catalyst 32 calculated based on a predetermined model equation including an operating state of the engine 10, a sensor value of the NOx/lambda sensor 45, or the like as input signals is reduced to a predetermined threshold indicating NOx removal success is satisfied, the NOx purge control is terminated by turning off the NOx purge flag F_(NP) (see time t₂ of FIG. 6).

In this manner, in this embodiment, the upper limit of the cumulative injection amount and the elapsed time is set in the termination condition of the NOx purge control so that it can be reliably suppressed that the fuel consumption amount is excessive in a case where the NOx purge does not succeed due to the lowering of the exhaust temperature and the like.

<MAF Follow-Up Control>

In (1) a period of switching from the lean state of a regular operation to the rich state through the SOx purge control or the NOx purge control, and (2) a period of switching the rich state to the lean state of the regular operation through the SOx purge control or the NOx purge control, the MAF follow-up controller 80 executes a control to correct the fuel injection timing and the fuel injection amount of the injector 11 in response to a MAF change (hereinafter, referred to the control as a MAF follow-up control).

<Injection Amount Learning Correction>

As illustrated in FIG. 9, the injection amount learning correction unit 90 includes a learning correction coefficient calculating unit 91 and an injection amount correcting unit 92.

The learning correction coefficient calculating unit 91 calculates a learning correction coefficient F_(Corr) of the fuel injection amount based on an error Δλ between an actual lambda value λ_(Act) detected by the NOx/lambda sensor 45 at the time of a lean operation of the engine 10 and an estimated lambda value λ_(Est). When the exhaust gas is in the lean state, the oxidation reaction of HC does not occur in the oxidation catalyst 31, and thus it is considered that the actual lambda value λ_(Act) in the exhaust gas which passes through the oxidation catalyst 31 and is detected by the NOx/lambda sensor 45 on the downstream side matches with the estimated lambda value λ_(Est) in the exhaust gas discharged from the engine 10. For this reason, in a case where the error Δλ occurs between the actual lambda value λ_(Act) and the estimated lambda value λ_(Est), the error can be assumed to result from a difference between an instructed injection amount and an actual injection amount in the injector 11. Hereinafter, the calculation process of the learning correction coefficient performed by the learning correction coefficient calculating unit 91 using the error Δλ will be described based on the flow of FIG. 10.

In Step S300, it is determined based on the engine speed Ne and the accelerator opening degree Q whether the engine 10 is in a lean operating state. If the engine 10 is in the lean operating state, the procedure proceeds to Step S310 in order to start the calculation of the learning correction coefficient.

In Step S310, a learning value F_(CorrAdpt) is calculated by multiplying the error Δλ, which is obtained by subtracting the actual lambda value λ_(Act) detected by the NOx/lambda sensor 45 from the estimated lambda value λ_(Est), by a learning value gain K₁ and a correction sensitivity coefficient K₂ (F_(CorrAdpt)=(λ_(Est)−λ_(Act))×K₁×K₂). The estimated lambda value λ_(Est) is estimated and calculated from the operating state of the engine 10 based on the engine speed Ne or the accelerator opening degree Q. The correction sensitivity coefficient K₂ is read from a correction sensitivity coefficient map 91A illustrated in FIG. 9 by using the actual lambda value λ_(Act) detected by the NOx/lambda sensor 45 as an input signal.

In Step S320, it is determined whether an absolute value |F_(CorrAdpt)| of the learning value F_(CorrAdpt) is in a range of a predetermined correction limit value A. In a case where the absolute value |F_(CorrAdpt)| exceeds the correction limit value A, this control returns to stop the present learning.

In Step S330, it is determined whether a learning prohibition flag F_(Pro) is turned off. The learning prohibition flag F_(Pro) corresponds, for example, to the time of a transient operation of the engine 10, the time of the SOx purge control (F_(SP)=1), the time of the NOx purge control (F_(NP)=1), and the like. It is because in a state where such a condition is satisfied, the error Δλ becomes larger according to the change of the actual lambda value λ_(Act) so that the learning is not executed exactly. As for whether the engine 10 is in a transient operating state, for example, based on the time change amount of the actual lambda value λ_(Act) detected by the NOx/lambda sensor 45, a case where the time change amount is larger than the predetermined threshold may be determined as the transient operating state.

In Step S340, a learning value map 91B (see FIG. 9) based on the engine speed Ne and the accelerator opening degree Q is renewed to the learning value F_(CorrAdpt) calculated in Step S310. More specifically, a plurality of learning areas sectioned in response to the engine speed Ne and the accelerator opening degree Q are set on the learning value map 91B. Preferably, such learning areas are set such that the range thereof is narrower as the area is used more frequently, and the range thereof is wider as the area is used less frequently. Accordingly, in the frequently used area, a learning accuracy can be improved, and in the less-frequently used area, non-learning can be effectively suppressed.

In Step S350, the learning correction coefficient F_(Corr) is calculated by adding “1” to the learning value read from the learning value map 91B by using the engine speed Ne and the accelerator opening degree Q as input signals (F_(Corr)=1+F_(CorrAdpt)). The learning correction coefficient F_(Corr) is input to the injection amount correcting unit 92 illustrated in FIG. 9.

The injection amount correcting unit 92 executes the correction of the fuel injection amount by multiplying respective basic injection amounts of a pilot injection Q_(Pilot), a pre-injection Q_(Pre), a main injection Q_(Main), an after injection Q_(After), by a post injection Q_(Post) by the learning correction coefficient F_(Corr).

In this manner, a variation such as the aged deterioration, the property change, or the individual difference of the injectors 11 can be effectively excluded by correcting the fuel injection amount of the injector 11 with the learning value according to the error Δλ between the estimated lambda value λ_(Est) and the actual lambda value λ_(Act).

<MAF Correction Coefficient>

The MAF correction coefficient calculation unit 95 calculates a MAF correction coefficient Maf_(corr) used to set the MAF target value MAF_(SPL Trgt) or the target injection amount Q_(SPR Trgt) at the time of the SOx purge control and to set the MAF target value MAF_(NPL Trgt) or the target injection amount Q_(NPR Trgt) at the time of the NOx purge control.

In this embodiment, the fuel injection amount of the injector 11 is corrected based on the error Δλ between the actual lambda value λ_(Act) detected by the NOx/lambda sensor 45 and the estimated lambda value λ_(Est). However, since the lambda is a ratio of air and fuel, a factor of the error Δλ is not necessarily limited to the effect of the difference between the instructed injection amount and the actual injection amount in the injector 11. That is, the error Δλ of the lambda may be affected by an error of the MAF sensor 40 as well as that of the injector 11.

FIG. 11 is a block diagram illustrating a setting process of the MAF correction coefficient Maf_(corr) performed by the MAF correction coefficient calculation unit 95. A correction coefficient setting map 96 is a map based on the engine speed Ne and the accelerator opening degree Q, and the MAF correction coefficient Maf_(corr) indicating the sensor property of the MAF sensor 40 corresponding to the engine speed Ne and the accelerator opening degree Q is set based on an experiment and the like, in advance.

The MAF correction coefficient calculation unit 95 reads the MAF correction coefficient Maf_(corr) from the correction coefficient setting map 96 by using the engine speed Ne and the accelerator opening degree Q as input signals, and transmits the MAF correction coefficient Maf_(corr) to the MAF target value calculation units 62 and 72 and the injection amount target value calculating units 66 and 76. Accordingly, the sensor property of the MAF sensor 40 can be effectively reflected to set the MAF target value MAF_(SPL Trgt) or the target injection amount Q_(SPR Trgt) at the time of the SOx purge control and the MAF target value MAF_(NPL Trgt) or the target injection amount Q_(NPR Trgt) at the time of the NOx purge control.

<Others>

The present invention is not limited to the above-described embodiment, and the invention may be modified appropriately without departing from the spirit and scope of the invention. 

1. An exhaust purification system comprising: an NOx reduction type catalyst, which is provided in an exhaust system of an internal combustion engine and reduces and purifies NOx in an exhaust gas; a temperature acquisition unit, which acquires a catalyst temperature of the NOx reduction type catalyst; and a regeneration treatment unit, which executes a catalyst regeneration to recover an NOx purification capacity of the NOx reduction type catalyst, wherein the regeneration treatment unit alternately executes a rich control, an exhaust air fuel ratio is set to a rich state to raise a temperature of the NOx reduction type catalyst to a predetermined target temperature, and a lean control, in which the exhaust air fuel ratio is set to a lean state to lower the temperature of the NOx reduction type catalyst, and sets an execution period of the lean control by a PID control, based on a deviation between the catalyst temperature acquired by the temperature acquisition unit during the previous rich control and the target temperature.
 2. The exhaust purification system according to claim 1, wherein the regeneration treatment unit sets an execution period of the rich control, based on an operating state of the internal combustion engine.
 3. The exhaust purification system according to claim 1, wherein the regeneration treatment unit executes the rich control by using a post injection or an exhaust pipe injection, and sets a post injection amount or an exhaust pipe injection amount based on an intake air amount of the internal combustion engine, a predetermined target excess-air-ratio, and a fuel injection amount of the internal combustion engine.
 4. The exhaust purification system according to claim 3, wherein the regeneration treatment unit sets a value, which is obtained by subtracting the fuel injection amount of the internal combustion engine from a value obtained by dividing the intake air amount of the internal combustion engine by the target excess-air-ratio, a fuel specific gravity, and a theoretical air-fuel ratio, as the post injection amount or the exhaust pipe injection amount.
 5. The exhaust purification system according to claim 3, further comprising: a lambda sensor provided in the exhaust system of the internal combustion engine; and an injection amount correcting unit, which corrects the fuel injection amount of the internal combustion engine based on a difference between an estimated lambda value estimated from the operating state of the internal combustion engine and an actual lambda value detected by the lambda sensor, wherein the regeneration treatment unit uses the fuel injection amount corrected by the injection amount correcting unit, as the fuel injection amount of the internal combustion engine. 